1. Field of the Invention
The present invention relates to a new process for producing polyvinyl ester having a high degree of polymerization and to a new process for producing polyvinyl alcohol having a high degree of polymerization.
More particularly, the present invention is concerned with a process for producing polyvinyl ester having a high degree of polymerization having an intrinsic viscosity not less than 1.5 dl/g, preferably not less than 2.2 dl/g, and most preferably higher than 3.2 dl/g, said process comprising subjecting vinyl ester monomer to emulsion polymerization under the following conditions (a), (b), (c), and (d).
(a) The medium is water or a mixture of water and (A) specified below; PA0 (b) The emulsifier is at least one member selected from (B), (C), and (D) specified below; PA0 (c) The initiator is a combination of (E) and (F) specified below or a combination of (E), (F), and (G) specified below; and PA0 (d) The polymerization temperature is not less than -60.degree. C. and not more than 15.degree. C. PA0 (A): At least one member selected from water-soluble alcohol, glycol, glycerin, or inorganic salts. PA0 (B): Nonionic emulsifier. PA0 (D): Anionic emulsifier. PA0 (E): At least one member selected from hydroperoxide, peroxide, or peracid ester. PA0 (F): Metal ion capable of transferring one electron. PA0 (G): Reducing substance. PA0 (1) polymerization is performed at a temperature substantially not less than 0.degree. C. PA0 (2) The use of .gamma. ray irradiation is liable to cause cleavage and cross-linking of the polymer. PA0 (3) The emulsifier is limited to special ones stable to .gamma. ray irradiation such as stearylamide propyldimethyl-.beta.-hydroxyethyl ammonium dihydrogen phosphate.
(C): Nonionic-anionic emulsifier.
The intrinsic viscosity of the polyvinyl ester is defined by a value measured in acetone at 30.degree. C. for polyvinyl acetate obtained by saponification, followed by reacetylation, of the polyvinyl ester.
The present invention relates also to a new process for producing polyvinyl alcohol (abbreviated as PVA hereinafter) having a high degree of polymerization, having an intrinsic viscosity not less than 1.5 dl/g, preferably not less than 2.2 dl/g, and most preferably higher than 3.2 dl/g, said process comprising saponifying in the usual way the polyvinyl ester obtained by emulsion polymerization under the above-mentioned conditions. (The intrinsic viscosity of PVA is defined by a value measured in acetone at 30.degree. C. for polyvinyl acetate obtained by reacetylation of the PVA.)
Polyvinyl ester, particularly polyvinyl acetate, finds use as a base polymer of adhesives and paints, and it is also very important as a starting material of PVA. PVA as one of the crystalline water-soluble polymers limited in number is used for paper processing, textile processing, and emulsion stabilization on account of its outstanding interfacial characteristics and mechanical properties. It is known that PVA is important as a raw material of vinylon film and vinylon fiber.
Conventional PVA, however, is limited in the degree of polymerization, with an upper limit of 2000 (and 3000 in the case of special product). This is because the PVA having a high degree of polymerization is hard to handle and poor in processability. The other reason is that it is difficult to produce polyvinyl acetate, as a starting material, having a high degree of polymerization.
The recent rapid innovation in processing technique has made possible the processing of polymers having an extremely high degree of polymerization. Such polymers have characteristic properties which have never been known. This is the case with PVA, too. PVA having a high degree of polymerization is expected to exhibit improved characteristic properties in the conventional application areas and is also expected to find new uses as high tenacity fibers and others.
2. Description of the Prior Art
In general, a polymer having a high degree of polymerization is obtained by polymerization at a low temperature and at a low polymerization rate. This is true of polyvinyl acetate, and several cases have been reported. (For example, A. R. Shultz; J. Am. Chem. Soc. 76 3422 (1954), G. M. Burnett, M. H. George, H. W. Melville; J. Polym. Sci. 16 31 (1955), and M. Matsumoto, Y. Ohyanagi; J. Polym. Sci. 46 148 (1960).)
The conventional processes are based on bulk polymerization in which polymerization solution is too viscous to carry out stirring. Therefore, in bulk polymerization, producing a uniform polymer and removing the heat of polymerization is very difficult. Therefore, the commercial production by bulk polymerization is considered to be almost impossible.
In order to eliminate the drawback of bulk polymerization, the present inventors proposed a new process by suspension polymerization. (Japanese Patent Laid-open No. 148209/1986)
According to the process based on suspension polymerrization proposed by the present inventors, the drawbacks of bulk polymerization are solved in most part. But, because in these processes with great chain transfer it is essential to lower the polymerization temperature and to lower the rate of polymerization for increasing the degree of polymerization.
Therefore, these proceses are not effective.
By contrast, emulsion polymerization is expected to provide a product having a high degree of polymerization at a comparatively high polymerization rate because it differs from bulk polymerization and suspension polymerization in polymerization mechanism. In addition, it has no trouble in stirring and heat removal which is inevitable to bulk polymerization. Therefore, emulsion polymerization is considered to be one of the effective polymerization processes for the production of polyvinyl ester having a high degree of polymerization and polyvinyl alcohol having a high degree of polymerization.
Canadian Patent No. 663.529 (1963) discloses a process for producing polyvinyl acetate having a high degree of polymerization and polyvinyl alcohol having a high degree of polymerization as a saponified product thereof by emulsion polymerization with .gamma. ray irradiation. This process is not necessarily a good process for the production of products having high degree of polymerization for reasons given below.
In general, the lower limit of polymerization temperature in emulsion polymerization is about 0.degree. C., because the medium (water) freezes, breaking emulsion particles, at a polymerization temperature lower than 0.degree. C. However, as mentioned earlier, it is necessary to perform emulsion polymerization at a low temperature, especially not more than 0.degree. C., in order to obtain polyvinyl ester having a high degree of polymerization and PVA having a high degree of polymerization. Therefore, the process disclosed in the above-mentioned Canadian Patent does not provide polyvinyl ester and polyvinyl alcohol having a sufficiently high degree of polymerization.